1. Field of the Invention
The present invention relates to an organic semiconductor polymer for an organic thin film transistor which contains quinoxaline rings in the backbone of the polymer. More particularly it relates to an organic semiconductor polymer for an organic thin film transistor in which quinoxaline rings having n-type semiconductor characteristics (e.g., high electron affinity) are incorporated into a polythiophene having p-type semiconductor characteristics, thereby simultaneously exhibiting both p-type and n-type semiconductor characteristics.
2. Description of the Related Art
Organic semiconductor materials for organic thin film transistors are largely divided into low molecular weight materials, e.g., pentacene, and high molecular weight materials, e.g., polythiophene. Although high molecular weight materials have poor device characteristics such as low charge carrier mobility, compared to low molecular weight materials, there is an advantage in terms of their easy processability: They can be processed in a large area at low costs by solution processing such as a printing technique. Since high molecular weight organic semiconductor materials can be formed into solutions, unlike the low molecular weight materials, they can be formed into thin films by screen-printing, ink-jet and roll-printing techniques. Such advantages of the high molecular weight organic semiconductor materials enable fabrication of large area transistors on plastic films at low cost.
Cambridge University and Seiko Epson Corp. have already fabricated and tested high molecular weight-based organic thin film transistor devices employing a polythiophene-based material, i.e. F8T2 (WO 00/79617 A1, Science, 2000, vol. 290, pp. 2132˜2126, charge carrier mobility: 0.01˜0.02 cm2/Vs). Bao. Z et al. from Lucent Technologies Inc. disclosed the fabrication of organic thin film transistor devices employing P3HT, which is a regioregular polymer (U.S. Pat. No. 6,107,117, charge carrier mobility: 0.01˜0.04 cm2/Vs). As noted above, these organic thin film transistors using high molecular weight materials have a low charge carrier mobility, compared to organic thin film transistors using pentacene, but do not require a high operating frequency and can be fabricated at low costs.
For commercialization of organic thin film transistors, important parameters of high Ion/Ioff ratio, as well as high charge carrier mobility, should be met. To meet these parameters, off-state leakage current is required to be reduced to the lowest possible extent. A representative regioregular polythiophene-based polymer, P3HT shows a charge carrier mobility of about 0.01 cm2/Vs, but has a low Ion/Ioff ratio of 400 or less due to its high off-state leakage current (10−9A or more) (U.S. Pat. Nos. 6,452,207 and 6,107,117). Recently, attempts have been made to improve these properties in various ways.
Research teams from Lucent Technologies Inc. reported an improvement in the physical properties of organic thin film transistor devices by using a combination of an n-type inorganic semiconductor material and a p-type organic semiconductor material as an active layer (U.S. Pat. No. 5,625,199). However, according to this patent, there is no particular difference from conventional silicon-based thin film transistor processes requiring deposition, which is not suitable for mass-production of thin film transistor devices.
T. Yamamoto et al. proposed an organic semiconductor polymer in which heteroaromatic rings having different electrical properties, such as quinoxaline rings are introduced into the backbone of the polymer, thereby simultaneously exhibiting both p-type and n-type semiconductor characteristics (J. Am. Chem. Soc. 1996, 118, 10389). Until now, no application of the polymer as an active layer of an organic thin film transistor has been reported because of low solubility in organic solvents and bad film formability.
Until now, high molecular weight-based organic thin film transistors that simultaneously satisfy the requirements of high charge carrier mobility and low off-state leakage current have not been reported.